This invention relates to agglomerates of iron oxide and lime which can be prepared in a form suitable to be fed into mixtures or melts for making iron and steel, and particularly, for example, as part of the feed for a blast furnace. In a special sense, the invention is concerned with making composites containing lime and iron values: (a) by utilizing material such as waste iron oxides that because of their finely divided state are not appropriate for direct supply to furnace operations; while (b) providing an effective and economical way of introducing lime that has already been calcined into such operations, where it is needed. Although the invention is concerned with iron oxides of any origin, including iron ore, the present improvements are particularly advantageous for use of iron oxide materials that might be deemed waste, as for example iron oxide fume or dust removed from the discharged gases of steel-making operation, and likewise scale or similar oxide developed from rolling, heat treating, or other steelmaking or processing steps.
It is manifestly desirable to recycle various iron oxides of the described character, for instance so as to use them in the primary process of making iron in a blast furnace. For such purpose, it is also desirable to get the waste oxides into useful physical shape, as in pellets or briquettes, for instance as has been done in the case of certain iron ores after magnetic or other concentration. It has also been recognized that iron oxide materials of this sort can be included along with iron ore in procedures for effecting combination of such ore with limestone to develop sintered, i.e. fused or partly fused products whereby both lime and iron values are combined in strong, durable pieces of agglomerate that can be used for furnace feed. The operation of making such sintered products is effective and the products themselves are useful for the intended purpose, but the sintering operation usually requires a relatively high temperature, e.g. to and above 2600.degree. F, and may involve environmental problems, e.g. resulting from the requisites as to nature of fuel and manner of combustion, with corresponding expense for correction of such problems.
In the sintered products just described, the limestone is converted to lime during the heating of the materials, and the lime is found to react with iron oxides (usually Fe.sub.2 O.sub.3 or Fe.sub.3 O.sub.4) at the sintering temperatures (2600.degree. and upwards) to yield dicalcium ferrite, a product from which the lime and iron oxide are readily released when the material is employed in a furnace.
A useful process for recovering iron oxide dust from furnace fume or similar gaseous sources has involved treating lumps of lime (e.g. as calcined in a preliminary step of the operation) to a flow of such iron oxide dust in furnace gas or the like. The temperature of the gas and thus of contact between the dust and the lime bodies is maintained in a range upwards of 2150.degree. F and the surrounding atmosphere (i.e. the gas) is caused to have a reducing nature such that the iron oxide is presented in the form of FeO, so-called wustite. Under such circumstances it is found, for example in a range of temperatures up to about 2600.degree. F, that the iron oxide is adsorbed into the surface layers of the lime lumps, where it reacts to form dicalcium ferrite, the remaining portion of the lime being unaffected and thus constituting a lime core of considerable size in each piece. In such process, where iron oxide particles were carried by the furnace gases to the lime lumps, the temperature of reaction was found quite critical, in that essentially no adsorption and corresponding chemical combination of the iron values into the lime occurred until temperatures substantially above 2100.degree. F, i.e. approaching 2200.degree. F, were reached.
In the light of the foregoing review of the prior art, it may be pointed out that the provision of waste iron oxides in the form of some kind of agglomerate is a desirable objective, and that another and eminently significant objective is to provide an economical and effective way of presenting lime, i.e. having the state of calcium oxide as distinguished from limestone, in a form that is practical, for the standpoint of storage and handling, for u se in blast furnaces. As will be explained below, there is an advantage in supplying lime to such furnaces in calcined or equivalent form, but lumps or other bodies of simple calcined lime, of size required for furnace feed, are not practical, because when stored in weather or ordinary air conditions, the material hydrates, and the pieces break up and deteriorate.